Essentially there are two major galvanizing processes in which a ferrous base metal strip is hot dipped into a molten zinc base metal. These two major galvanizing processes are referred to as the "Sendzimir" process and the "non-oxidizing" process.
In the "Sendzimir" process (see U.S. Pat. No. 2,110,893 to Sendzimir), a ferrous base metal strip is first introduced into an oxidizing furnace, which burns off any oil or organic material on the strip, and simultaneously oxidizes the strip to form a surface coating of metal oxide (primarily ferrous oxide). Next, the ferrous base metal strip is introduced into an enclosed, sealed, reducing furnace, which reduces the oxides on the surface of the strip, leaving a cleaned strip which is maintained in an enclosed snout containing a protective reducing atmosphere generally including hydrogen, and nitrogen and/or other inert gases. Lastly, the ferrous base metal strip is hot dipped in a molten zinc based coating bath in which the excess zinc based coating on the exiting strip is removed by a pair of wiping or coating rolls, generally positioned at or slightly above the molten coating bath surface.
A "non-oxidizing" process is taught in U.S. Pat. No. 3,320,085 assigned to the Selas Corporation. Any oil or dirt on the ferrous base metal strip is removed by a washing or pickling process, followed by a water rinse, which leaves a substantially invisible oxide film on the surface of the strip. The ferrous base metal strip is then introduced into a reducing furnace to remove the oxide coating. The reducing furnace is heated by the direct combustion of fuel and air to a temperature of at least 2400.degree. F., wherein the combustion atmosphere has no free oxygen and at least 3% excess combustibles. From the direct combustion furnace, the cleaned ferrous base metal strip is generally maintained in an enclosed snout containing a protective atmosphere such as hydrogen, nitrogen or other inert, non-oxidizing gases. Lastly, the ferrous base metal strip is hot dipped in a molten zinc based coating bath in which the excess zinc based coating on the exiting strip is removed by a pair of wiping or coating rolls, generally positioned at or slightly above the molten coating bath surface. Several major problems are encountered with these prior art galvanizing processes. The most important problem area is that of coating control of the ferrous base metal strip including non-uniform coating, edge berries, spangled relief, and feathered oxides.
Except for the portion of the surface of the molten zinc base bath which is within the enclosed snout, the remainder of the molten metal surface is ordinarily exposed to the atmosphere in the prior art processes. Accordingly, a layer of dross, which is primarily zinc oxide, is formed on the exposed portion of the molten metal surface. The dross is a metal oxide characterized by bits of flaky solid material.
Bits of dross are picked-up by the ferrous base metal strip, particlarly at the end edges of the strip, as the strip exits the molten metal coating pot. The bits of dross are called edge berries.
Edge berries cause two problems. The first problem concerns those edge berries which are not removed from the strip by the coating rolls, and thus end up on the galvanized strip. The second problem concerns those edge berries which are transferred from the ferrous base metal strip to the coating rolls, thus yielding a non-uniform coating on the strip for each revolution of the coating rolls caused by the non-uniform surface of the coating rolls.
Edge berries were greatly diminshed by the use of jet finishing knives in place of the wiping rolls as taught in U.S. Pat. No. 4,137,347. The jet finishing knives may be positioned about 0.5 to 4.0 feet above the surface of the molten zinc base metal bath, and direct pressurized air at both sides of the ferrous base metal strip. As the bits of dross are picked-up by the strip, the jet finishing knives sweep away the excessive coating and most of the bits of dross or edge berries. Nevertheless, some edge berries still adhere to the ferrous base metal strip, causing the previously mentioned first problem.
A spangle is a zinc crystal usually easily visible on some galvanized ferrous base metal strips. Spangle relief concerns both the variation in zinc thickness across the zinc crystal and a depressed spangle boundary which surrounds each crystal. Thus, a non-uniform thickness or coating results if the spangles are prevalent and large in size. Spangle relief can be greatly eliminated by permitting the iron and zinc to alloy, forming a galvanized strip whose inner layer is iron, and an intermediate layer of alloyed iron and zinc, with an outer layer of zinc. However, both zinc and iron are ductile, while an iron-zinc-alloy is brittle. Accordingly, the brittle layer may flake-off the ductile iron layer if the alloy layer is too thick and is work-stressed, such as by sharp bending.
Another procedure to reduce spangle relief is to add antimony to the molten zinc base metal which changes the crystal morphology, resulting in smaller crystals, thereby minimizing the size of the spangle and resulting in a more uniform thickness of the spangle. However, neither of these methods is entirely satisfactory because the results are not consistent.
If the ferrous base metal strip is pulled through the jet finishing knives at low speeds, care must be taken to avoid causing the zinc metal to oxidize, thus forming a metal oxide film on the coating surface. This problem is termed "feathered oxides" because they appear like feathers which extend inwardly toward the center of the strip.
These problems of non-uniform coating, edge berries, spangle relief and feathered oxides were cured by maintaining a non-oxidizing or inert gas atmosphere within a coating chamber mounted around and above where the ferrous base metal strip exits from the surface of the molten coating. Molecular oxygen is maintained at less than 1000 ppm in the coating chamber (see U.S Pat. No. 4,330,574 to Pierson et al). For best results, the molecular oxygen was maintained at below 100 ppm within the coating chamber, and preferably below 50 ppm. By employing a non-oxidizing or inert gas for the jet finishing knives, and surrounding the freshly coated strip and jet finishing knives with a coating chamber, a positive pressure can be maintained within the chamber, which will prevent the formation of zinc oxides as dross, edge berries and feathered oxides. Furthermore, for some unexplained reason, spangle relief is greatly diminished and the spangles are much more uniform in size and thickness.
A non-oxidizing or inert gas has also been employed in a one side coating process of a ferrous base metal strip as exemplified by U.S. Pat. No. 4,114,563 to Schnedler et al. As disclosed therein, the uncoated strip travels sufficiently close to the surface of the molten metal to cause the formation of a meniscus which continuously contacts and coats one side of the ferrous base metal strip. Once one side of the strip is coated, a jet finishing knife is employed to remove the excessive coating. The strip, both before and immediately after being coated, is protected by an enclosure maintained with a positive pressure of non-oxidizing or inert gas. After coating, the strip preferably remains in the enclosure until it has sufficiently cooled and solidified to prevent the coating from oxidizing before it bonds with the strip.
U.S. Pat. No. 3,383,250 teaches a coating process wherein one side of the ferrous base metal strip is oxidized, which prevents the coating material from adhering to the oxidized side. The strip is totally submerged in the molten metal to yield a one side coated strip. Subsequently, the strip is subjected to a cleaning procedure to remove oxide on the uncoated side of the strip.
It is also known that one side of the ferrous base metal strip can be physically or chemically masked (other than by oxidation) such as by employing a film of calcium base slurry. The strip is then totally submerged to coat the unmasked side and subsequently the physical or chemical mask is removed.
Although the non-oxidizing or inert gas atmosphere in the coating chamber solves the many problems previously mentioned, a severe new problem developed when using a process like the Pierson et al or Schnedler et al process. That problem is the undesirable formation of zinc vapor. The zinc vapor leaks from the coating chamber and creates a potentially adverse environmental condition. The vapor condenses and "zinc dust" coats the surrounding work area.
It is theorized that the reduction of oxygen in the Pierson et al or Schnedler et al process results in zinc vapor becoming the dominant partial vapor pressure within the coating chamber, thus substantially increasing the formation of zinc vapor. The following two prior art references recognize the problem of zinc vapor formation and attempted to reduce its escape into the work environment:
U.S. Pat. No. 4,369,211 to Nitto et al recognizes the zinc vapor problem and proposes a solution to zinc vaporization by maintaining an oxygen controlled atmosphere from 50 to 1000 parts per million in the coating chamber to diminish or eliminate the zinc vapor formation. Also, Nitto et al state that it is essential that the zinc base alloy coating contain 0.1 to 2% by weight magnesium to inhibit surface corrosion on a coated metal strip. It is theorized that molten magnesium in the hot dip coating may exert some influence upon the formation of zinc vapor, once a low oxidizing-potential atmosphere has been achieved, and consequently, it is alleged that both the magnesium in the hot dip coating, and the maintenance of a minimal amount of molecular oxygen in the atmosphere of the coating chamber help reduce or eliminate the formation of the zinc vapor.
For high speed coating lines the process of Nitto et al is insufficient because steady-state conditions are difficult to maintain by controlling the atmosphere within the coating chamber from 50 to 1000 ppm. Although some improvement in controlling zinc vapor formation may exist, substantial zinc vapor continues to form and create the previously described coating and environmental conditions.
Belgion Pat. No. 887,940 to Heurtey also recognizes the formation of the zinc vapor in the entry section to the coating pot. It eliminates the passage of zinc vapor into the cooling and furnace equipment which are positioned before the coating pot by employing a sweep gas which sweeps over the hot dip bath surface, becoming loaded with the coating metal vapor, and is then evacuated and further treated to condense the coating metal, thus preventing the transfer of the zinc vapor to other parts of the installation. This patent does not attempt to control the zinc vapor formation by any particular atmosphere and moreover, it does not control the zinc vapor formed within the coating chamber.
Because the Nitto et al process is insufficient for high speed coating and requires the addition of magnesium in the coating pot, and because the Belgian process is not practical in that additional equipment necessary to extract the zinc vapor from the sweep gas is required, there is a need for controlling the atmosphere within the coating chamber that is both inexpensive, requiring simple equipment, and operable by a minimally skilled technician.